A) Field of the Invention
The present invention relates to a liquid crystal display apparatus, and more particularly to a liquid crystal display apparatus having a vertical alignment type liquid crystal cell.
B) Description of the Related Art
A “vertical alignment type” liquid crystal display (LCD) device, whose liquid crystal molecules in a liquid crystal layer of a liquid crystal cell are aligned vertical to a substrate, shows a very good black level during no voltage application. Very good view angle characteristics are obtained when an optical compensation plate having negative optical anisotropy with proper parameters is disposed between the liquid crystal cell and at least one of the upper and lower polarization plates.
One type of the vertical alignment type liquid crystal display device is a mono domain vertical alignment type liquid crystal device. Alignment of the mono domain vertical alignment type liquid crystal device is controlled to have a uniform alignment state in the liquid crystal layer, independently from whether a voltage is applied or not. In order to prevent alignment defects during voltage application, it is necessary to set a pretilt angle so that liquid crystal molecules slightly tilt from the vertical to the substrate even during no voltage application.
An alignment control method of this type includes a “metal oxide oblique vapor deposition method” of vapor-depositing an SiOx film along an oblique direction to the substrate, on an alignment film disposed on inner surfaces of the upper and lower substrates, a method of coating a polymer alignment film such as polyimide on a substrate and then performing a rubbing process, and other methods. The rubbing process is advantageous in terms of production. However, a rubbing process similar to that in manufacture processes for a twisted nematic (TN) type liquid crystal display device, may often form stripe flaws along a rubbing direction, posing a fear of degrading considerably a display quality.
The present inventor and his colleagues have proposed alignment process techniques suppressing flaws during rubbing in JP-A-2005-234254, the entire contents of which are incorporated herein by reference. A method disclosed in this Publication can realize a mono domain vertical alignment type liquid crystal display device capable of suppressing flaws and having a pretilt angle of, e.g., 88.5° to 89.5°.
Another type of the vertical alignment type liquid crystal display device is a multi domain vertical alignment type liquid crystal display device. The multi domain vertical alignment type device has a plurality of liquid crystal molecule orientations in one pixel so that the view angle characteristics of the display device can be improved during voltage application.
As a multi domain alignment control method, for example, JP-A-HEI-3-259121 and Japanese Patent Publication No. 3834304, the entire contents of both documents are incorporated herein by reference, propose an “oblique electric field alignment control method” of forming rectangular openings through partial areas of electrodes formed on inner surfaces of upper and lower substrates, constituting pixels and controlling alignment orientations of liquid crystal molecules by oblique electric fields formed near the openings during voltage application.
One of a driving method for a vertical alignment type liquid crystal display device is a multiplex driving method. The summary of main specific features of a present (direct) multiplex driving method is explained, for example, in a document “An Electrical Driving Method for LCD”, written by Takashi SUGIYAMA and Shunsuke KOBAYASH, in Magazine: Display and Imaging, pp. 117 to 131, Vol. 3, 1994 published by Science/Communications/International.
The most general driving method is an “optimum biasing method”. An electro-optic response of a liquid crystal display is determined by an effective voltage value, and an alternate current drive (an average voltage is 0) is fundamental because deterioration of the device performance can be prevented. Driving waveforms realizing this include: “intra-frame reversal driving (or one line reversal driving)” of reversing a polarity during one line selection as shown in FIG. 9A (hereinafter this driving waveform is called waveform A); “frame reversal driving” of reversing a polarity at each frame as shown in FIG. 9B (hereinafter this driving waveform is called waveform B); and “N-line reversal driving” which is based on waveform B and reverses a polarity at N-lines to reduce crosstalks (in the above-described explanatory document, the second kind crosstalks) in high duty driving as shown in FIG. 9C (hereinafter this driving waveform is called waveform C).
FIGS. 9A to 9C show driving waveforms applied across upper and lower electrodes of one pixel. Waveform B having the lowest consumption power during driving is widely used for the present multiplex driving LCD.
A method of suppressing a phenomenon “frame response” which occurs when a response speed of a liquid crystal display device is high includes: an active addressing method by which a plurality of select times are assigned to one frame; and “a multi line simultaneous select method” (hereinafter this driving waveform is called waveform MLS) of selecting N lines in one frame at the same time as shown in FIG. 12, disclosed, for example, in Japanese Patent Publication No. 3119737. The latter method is often used for driving a high speed response STN-LCD having a duty ratio under 1/16 (a duty number is larger than 16).
FIG. 12 shows an example of 2 lines simultaneous select drive (2-line MLS) waveform at 1/16 duty ratio drive. This waveform corresponds to the case that both 2 lines are on-state. There are 2 levels for segment voltage VL0 and VL1 when 2 lines are simultaneously selected. The voltage VL0 is 0V.